There are many medical diagnostic tests that require a blood sample. A venous blood sample is usually collected in a vacuum-filled tube and taken to a central laboratory for analysis. In most cases the venous blood has to be centrifuged to obtain plasma, and the plasma is tested. In circumstances where arterial blood is needed, the blood is collected in a syringe from an artery or an arterial line (i.e., a tube connected to an artery), and the blood is taken to a central laboratory for analysis. Alternatively, much smaller blood samples (e.g. in the range of micro-liters) can be obtained using a pinprick and then a capillary tube that is inserted into a drop of blood that oozes onto the skin surface from the pin prick. Blood from the drop flows into the capillary tube as a result of capillary action. Blood from a pin prick flows out of capillaries, and hence is called capillary blood.
Babies cannot always provide an arterial blood sample, because the blood loss can affect their health. As a substitute, capillary blood can become “arterialized” by applying a heating pad to a baby's skin at the site chosen for the pinprick. The heat increases the blood flow in the area and the resulting capillary blood is similar in composition to arterial blood.
Point-of-care testing or near-patient testing is a process of testing the patient's blood near the patient. Point-of-care testing has many advantages, but analyzers that provide point-of-care testing are only available for a limited number of tests.
One example of a blood analysis technique that requires arterial blood or “arterialized” capillary blood is co-oximetry. Co-oximetry is a spectroscopic technique that can be used to measure the different Hemoglobin (Hb) species present in a blood sample. The results of co-oximetry can be further evaluated to provide Hb Oxygen Saturation (sO2) measurements. Preferably, Hb sO2 is measured from arterial blood, since arterial blood provides an indication of how well venous blood is oxygenated in the lungs. If the blood sample is exposed to air the Hb sO2 measurements are falsely elevated, as oxygen from the air is absorbed into the blood sample. Moreover, the presence of small air bubbles trapped inside the capillary tube also lead to analysis errors, because the partial pressure of oxygen in the sample rises. Evidence of this is found in the Tietz Textbook of Clinical Chemistry, 3rd ed. (ISBN: 0721656102); which describes a representative example of how a 100 micro-liters air-bubble causes a 4 mm of mercury increase in the partial pressure of oxygen in a 2 ml blood sample. It is commonly understood that this effect increases as the ratio of blood sample volume to air volume decreases.
A sample holder referred to as a “Sample Tab” is described in U.S. Pat. No. 6,841,132 and U.S. Pat. No. 7,108,833 for use in point-of care testing. The Sample Tab, which comprises a well and a hinged-cover, can also be used in the central laboratory. The major drawback of the Sample Tab is that the blood is exposed to the atmosphere, and consequently cannot be used to measure blood oxygenation. Also, the well of the Sample Tab is difficult to fill when the blood comes directly from a pinprick. The present invention overcomes some of the limitations of the Sample Tab.